1. Field of the Invention
This invention relates to a heat-sensitive recording sheet and particularly to a heat-sensitive recording sheet having excellent image stability.
1. Description of the Prior Art
In recent years, the heat-sensitive recording method has come to possess various advantages such that it gives almost no impact and accordingly is noiseless, image records are obtained without development and fixation works and equipment maintenance is simple. Therefore, the method is widely used not only in various printers and telephone facsimile but also in many other areas. In the field of the telephone facsimile, the heat-sensitive system has been popularized sharply, which is being improved to a faster speed type for reduction of transmission cost. To respond to the speed-up of facsimile, an increased sensitivity has come to be required for heat-sensitive recording sheets. Further, retention of image records over a long period of time is strongly required and in this connection colored dyes must not cause fading and/or discoloration during retention of image records under moisture, heat and other conditions or through hand touch to image records.
Hitherto, as stabilizers for improving the retention of image records, phenol type oxidation inhibitors are described in Japanese Patent Application Kokai (Laid-open) No. 45747/1974, Japanese Patent Application Kokai (Laid-open) No. 18752/1979 and Japanese Patent Application Kokai (Laid-open) No. 83495/1982. However, none of these stabilizers can satisfactorily improve image stability without lowering sensitivities of heat-sensitive recording sheets.